A leading chemist says that a better understanding of photosynthesis could lead to cheap ways to store solar energy as chemical fuel.
While researchers and technologists around the world scramble to
find cleaner sources of energy, some chemists are turning to nature's
own elegant solution: photosynthesis. In photosynthesis, green plants
use the energy in sunlight to break down water and carbon dioxide. By
manipulating electrons and hydrogen, oxygen, and carbon atoms in a
series of complex chemical reactions, the process ultimately produces
the cellulose and lignin that form the structure of the plant, as well
as stored energy in the form of sugar. Understanding how this process
works, thinks Daniel Nocera,
professor of chemistry at MIT, could lead to ways to produce and store
solar energy in forms that are practical for powering cars and
providing electricity even when the sun isn't shining.
What's needed are breakthroughs in our understanding of the
fundamental chemical processes that make photosynthesis possible,
according to Nocera, a recognized photosynthesis expert. He is studying
the principles behind photosynthesis and applying what he learns to
making catalysts that use solar energy to create hydrogen gas for fuel
cells. Nocera's goal: a world powered by light and water.
Technology Review: What's the biggest challenge related to energy right now?
Daniel Nocera: The real challenge with energy is
the scaling problem. We're going to have this huge energy need, and
when you start looking at all the numbers, there's only one supply that
has scale, and it's the sun. But it's still a research problem.
Technologies all follow lines; then there's a discovery and a new line
that's better. We're on a very predictable line now in solar. Most
things you hear about are incremental advances.
TR: You're studying photosynthesis to get
ideas for how to convert sunlight into a chemical fuel--hydrogen--for
use when the sun isn't shining or in powering fuel-cell vehicles.
DN: You can use the electricity directly when the
sun is out, in places that have sun. [But] you need storage. There's
absolutely no way around it. I am distilling the essence of
photosynthesis down to be able to use it.
TR: Why is photosynthesis attractive in finding a source of clean energy?
DN: [Photosynthesis] does three things. It captures
sunlight, and [second,] it converts it into a wireless current--leaves
are buzzing with electricity. And third, it does storage. It stores the
converted light energy in chemical energy. And it uses that chemical
energy for its life process, and then it stores a little.
It turns out [that] photosynthesis is one of the most efficient
machines in the world for energy conversion. But it's not great for
storing energy because that's not what [a plant] was built to do. It
was built to live and grow and reproduce.
And so that's the approach we take. Can we now do what the leaf is
doing artificially, which is the capture, conversion, and storage in
chemical bonds? But my device doesn't have to live: it can take a lot
more of that energy and put it into chemical bonds.
TR: And you've had some success putting what you've learned to use.
DN: We did make a compound that makes hydrogen
using light. We have something that you can dissolve in solution, shine
light on it, and hydrogen comes bubbling up. It didn't do it that
efficiently. But it was a big advance because it had a lot of new
concepts in there to show how you can use sunlight to make hydrogen.
TR: What are some of the research problems you're addressing that you hope can lead to a big step forward in solar?
DN: Something we've really been working hard at is [understanding] the design principles that photosynthesis operates off.
One is that when [photosynthesis] splits water into hydrogen and
oxygen, it uses more than one electron. This current that's running is
going one electron at a time. But then [the plant] stores them and uses
four electrons at once. We don't know how to do multi-electron
reactions very well. We don't even have theories to describe them.
And then you have to manage protons--and that's what biology does
really well. It takes electrical current and then it converts it to a
chemical current, and the thing conducting the chemical current is
protons. And then it sends atoms. What a photovoltaic does is send
electrons to a point. Photosynthesis actually sends not an electron but
an atom. And that's even a tougher thing to do because atoms are so
much heavier than electrons. So we've gotten down deep into
understanding, how do you move atoms [such as hydrogen atoms] around
from point A to B so that they can join up with each other? How do you
assemble them so they can unite?
TR: You've written that chemistry "will
likely play the most central role of all the sciences" in addressing
energy problems. How would you summarize the role of chemistry?
DN: For game changers, it's really easy. There's three.
Make photovoltaics cheap, which is a lot of chemistry. It's inventing new materials to make PV cheap.
Replace noble metals--things like platinum--with abundant metals.
Because there's not enough stuff. When you're talking about this much
scale, you better be using things like iron and manganese. You better
look at your book that says what are the most abundant elements on the
face of the earth.
TR: And this is for fuel cells, and also for photovoltaics.
DN: Photovoltaics--everything. That's the real
technology issue that you have to keep in your mind. Not something
that's so great, it's 100 percent efficient--and oh, by the way, I'm
using ruthenium. I can use ruthenium now to teach me a principle, but
ruthenium's below iron [on the periodic table]. So I better figure out,
how can I take everything I'm learning with ruthenium and apply it to
iron?
TR: And the third game changer?
DN: Split water with light. You do those three
things, and you have a full new energy economy. It's hard for me to say
exactly what that technology will look like, because the science is
missing. But at the beginning of the 1900s, we built an entire society
based on a new energy system. And I believe, once solar is in place,
with help from biofuel, with a little help from wind, we will invent
our society again from a new energy source.
Source:
http://www.technologyreview.com/Energy/18707/
